Field of the Invention
The present invention relates to a sheet processing apparatus which creates a booklet by saddle-binding and half-folding a sheet bundle including a plurality of sheets.
Description of the Related Art
Conventionally, there is known a sheet processing apparatus which creates a booklet by binding a sheet bundle by a saddle-binding processing unit, and performing a folding process by a folding processing unit. The booklet created in this manner has a problem that a crack occurs at the folded part of the outer sheet when the sheet bundle is thin, and the created booklet opens when the sheet bundle is thick.
The crack occurring at the folded part is a phenomenon in which the folded part of the outer sheet cracks because the elongation amount of the outer sheet of the sheet bundle becomes larger than the elongation amount of the inner sheet when the folding process is performed. This phenomenon easily occurs when the sheet being used is thick, and the crack occurring at the folded part of the outermost sheet of a booklet deteriorates the appearance. In order to prevent this, there is known a sheet scoring apparatus which prevents occurrence of a crack at a folded part by scoring a sheet at the folded part in advance as disclosed in Japanese Patent Laid-Open No. 2011-57363.
In addition, the reason that a booklet opens is that a sheet bundle is thick and folding is not enough. In order to prevent this, there is known an additional folding unit called a square fold which flatly crushes a folded part as disclosed in Japanese Patent Laid-Open No. 2007-144679.
However, scoring is not effective in every booklet. The scoring in which a sheet bundle is firmly folded is effective in a thin sheet bundle. However, folding to a thick sheet bundle in which cracks hardly occur at the folded part is not enough and the scoring merely produces a booklet having a score at the folded part. Although the thick booklet is subjected to square fold to be strongly folded by flattening the folded part, the booklet having a score on the flattened spine is created. For this reason, it is necessary to perform a troublesome work for operator to create a booklet to determine whether scoring a sheet bundle is necessary.